The below mentioned article provides notes on elements of cytoskeleton.
Cytoskeleton is shown to be present in both pro- and eukaryotic cells and are responsible for the stability of cell shape and different movements within and among the cells.
Plasma membrane is associated with a cytoskeleton called ‘membrane skeleton’ and hence regulates several functions of the membrane.
Basically cytoskeleton comprises three elements within a cell and these are:
i. Network of microtubules,
ii. Actin filaments or microfilaments, and
iii. Intermediate filaments.
We shall briefly describe the role of these cellular structures.
Microtubules:
Except in amoeba, slime molds and mature mammalian erythrocytes, their presence is ubiquotous.
They also occur in cilia and filaments, centrioles and basal bodies, nerve processes etc. These are tube like hollow cylindrical structures unbranched and are several microns in length, and seem to comprise 13 rows of subunits.
Each subunit is circular in diameter and is called protofilament being helically arranged around a central axis. (Fig. 3-14).
Chemically each subunit is made up of two globular polypeptides called A tubulin and β tubulin.
When both the protein dimers are associated they form tubulin. The latter has kinase and ATPase activities.
With tubulins some proteins are associated and these are called microtubule associated proteins.
Microtubules have one end being plus (+) and the other as minus (-). The minus end remains embedded in centrosome and is stabilized while the plus end is free to grow.
Microtubules elongate or shrink. They have diverse functions: cell division, chromosome segregation/movement, etc.
Actin Filaments:
Actin consists of long fibres of variable length each with a diameter of 5-7 nm and has protein molecules. They form web in the ectoplasm and are devoid of ER, GB, mitochondria, ribosomes, chloroplasts.
They play divergent roles: formation of cell plate after mitotic division; cytoplasmic streaming; cell migration during early embryogenesis.
Actin filaments are destroyed by cytoclasin B. Micrifilaments are involved in the growth of pollen tubes.
Intermediate filaments:
These filaments are intermediate between actin and microtubules as regards their thickness.
They were discovered in hair proteins in 1950 and their half-life is one hour or even less. Each IF consists of three or four protofibrils, each with two protofil aments (Fig. 3-14A).
They are of four types: eratin filaments; neurofil aments; glial filaments and hetergenous filaments. All the four types have divergent functions. Intermediate filaments may interact with microtubules during motility.
